This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-279745, filed Sep. 25, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which uses a magneto-resistive effect element as a storage element.
2. Description of the Related Art
In recent years, research and development of a magnetic random access memory which stores data in a non-volatile manner by utilizing a magneto-resistive effect have been frequently carried out.
The magnetic random access memory stores data xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d by using a magneto-resistive effect element (a TMR element, a GMR element or the like). The basic structure of the magneto-resistive effect element is a structure sandwiching a tunneling barrier by two magnetic layers.
Data stored in the magneto-resistive effect element is judged by whether directions of magnetization of the two magnetic layers are the same (parallel state) or opposite to each other (anti-parallel state).
Here, an anti-magnetic layer for fixing a direction of magnetization of a fixed layer is brought into contact with one (fixed layer) of the two magnetic layers. As a result, data to be stored in the magneto-resistive effect element is actually determined based on a direction of magnetization of the other one (free layer) of the two magnetic layers.
When magnetization of the magneto-resistive effect element enters the parallel state, the tunnel resistance of the tunneling barrier sandwiched between the two magnetic layers constituting the magneto-resistive effect element becomes lowest. This state is, for example, the state xe2x80x9c1xe2x80x9d. Further, when magnetization of the magneto-resistive effect element enters the anti-parallel state, the tunnel resistance of tunneling barrier sandwiched between the two magnetic layers constituting the magneto-resistive effect element becomes highest. This state is, for example, the state xe2x80x9c0xe2x80x9d.
In the magnetic random access memory, a write current is used when programming data. The write current is caused to flow through two write lines crossing each other at a point where the magneto-resistive effect element is positioned. The state of magnetization of the magneto-resistive effect element is controlled by a combined magnetic field generated by the write current flowing through the two write lines.
In the conventional magnetic random access memory, however, an address decoder may become unstable when turning on a power supply in some cases. In this case, an unexpected row/column address is selected, a write current flows through a write word/bit line corresponding to that row/column address, and data in an arbitrary cell may be possibly destroyed by erroneous writing.
Such a phenomenon becomes a problem when using the magnetic random access memory as a ROM in particular.
A magnetic random access memory according to an aspect of the present invention comprises: a memory cell array determining an element having a magnetic layer as a memory cell; a first write line for writing data in a memory cell; a first decoder which selects the first write line based on a first address signal; and a first driver which is connected to one end of the first write line and supplies a write current to the first write line when the first write line is selected by the first decoder, and the first decoder is maintained in a non-active state until an initialization operation of an internal circuit is completed after turning on a power supply.